Sim card transfer question

I just decided to make sure my new Galaxy S4 phone and sim card both are saving my contacts (got the phone about 3 weeks ago and am VERY smartphone challenged; have been using an iPhone 4 for 3 years). Only my phone is saving them so when I clicked on "copy to sim", it says "Some information might be lost. Continue?" Does anyone know what that info might be? And is there a setting to automatically add contacts to both the phone and the sim?
Sorry if this seems like a dumb question but I'm in my 60's and just don't have a very good relationship with electronics. I ordered it online and took it and my iPhone into one of their retail stores and had them set it up for me. They were incredibly busy and the sales rep really rushed through just a few basics. This phone is much more high tech than my iPhone. I researched reviews all over the internet and it consistently got great reviews. But I've been going back and forth between the regular and easy modes (embarrassing, I know).
Thanks!
Lisa

Lisa, smartphones are such complex devices that I don't suppose there is a single member of these forums who doesn't have some trouble understanding them, so you are not alone!

Regarding the partial loss of information, I think that the phone saves a sort of trimmed down version of the information for each contact...just the name and number or something, because of the limited storage capacity of the SIM card.

Here's a link to a website that may help to see what's been saved to your SIM:

Also, you may care to consider opening a Google account if you haven't already...you can then save or back up your contacts and other personal data to that. You can save more that way, than you can to the limited storage on a SIM card.

Hello and Welcome to Android Central.
The S4 is a great phone to start on. Like my first Android 2 years ago, I find the menu systems generally helpful and comprehensive, although it still takes a while to discover everything.
I think generally sim cards will only save the name and phone number, as in days of yore, whereas nowadays you can input their address, email, cat's name, whether you like them or not etc. so this is probably what it means about losing information.
Sim cards have a very limited memory.
As you get more familiar with how your phone and Google/android works, you will learn your contacts will be saved to your Google account unless you don't want it to, to your device (as in phone) to your Samsung account if you bother, and you can still save your main numbers to the sim card (up to 200 I think). There is a menu option when you open Contacts for Contacts to Display and you can set it to sim only if you like, but you will just have a name and number. You can choose to merge all lists so they are all collected.
Enjoy your S4, and come back and see us here, in Introductions, or the S4 forums when you need more information, and you will! We all did.